Performance glossary
Plain-English definitions of the web-performance metrics and PageSpeed terms we use — LCP, CLS, INP and the rest.
Need the numbers themselves? How to run a Lighthouse speed test — three ways to do it, and what to make of the fact that the score moves between runs.
Core Web Vitals 4
CLS — Cumulative Layout Shift
A score for how much visible content unexpectedly jumps around while the page loads — one of Google's three Core Web Vitals.
Core Web VitalCore Web Vitals
Google's three field metrics for real-world loading, responsiveness and visual stability — LCP, INP and CLS — that feed into search ranking.
Core Web VitalINP — Interaction to Next Paint
How quickly a page visibly responds to taps, clicks and key presses across the whole visit — the Core Web Vital that replaced FID in March 2024.
Core Web VitalLCP — Largest Contentful Paint
The time it takes for the largest image or text block in the viewport to render — one of Google's three Core Web Vitals.
Lab metrics 5
FCP — First Contentful Paint
The moment the browser renders the first piece of real content — the first sign to the visitor that the page is loading.
Lab metricSI — Speed Index
How quickly the visible part of the page fills in during load, averaged over time — a lab metric that rewards showing content sooner.
Lab metricTBT — Total Blocking Time
The total time the main thread was blocked by long tasks during load — the lab metric that stands in for real-world responsiveness.
Lab metricTTFB — Time to First Byte
How long the browser waits before the server sends the very first byte of the page — the delay that sits under every other loading metric.
Lab metricTTI — Time to Interactive
When a page becomes reliably responsive to input — a legacy Lighthouse metric now largely superseded by TBT and INP.
Tools & concepts 13
Conversion rate
The share of visitors who take the action you want — buy, sign up, submit a form. It's the business metric page speed most directly moves, which is why a faster site is worth measuring in revenue, not just milliseconds.
ConceptCritical CSS
The small slice of CSS needed to render the first screen, inlined into the HTML so the page can paint without waiting for a stylesheet to download.
Conceptfetchpriority
The HTML attribute that tells the browser which downloads matter most — the one-line fix for a hero image queued at the same priority as a footer icon.
ConceptField data
Measurements collected from real visits rather than one simulated load — the distinction that decides which of the two conflicting numbers on your PageSpeed report is the verdict.
Conceptfont-display
The CSS descriptor that decides what a visitor reads while a web font is still downloading: nothing, a fallback typeface, or the fallback kept permanently.
Conceptgzip and Brotli — gzip, Brotli and Zstandard text compression
The compression algorithms a server applies to HTML, CSS and JavaScript before sending them, declared in the Content-Encoding header. Brotli is usually smallest, gzip is universal, Zstandard is the newest of the three.
ConceptLazy loading
Deferring the load of off-screen images, iframes and scripts until they're about to be needed, so the browser spends its first moments only on what the visitor can actually see.
ToolLighthouse
Google's open-source engine that runs a page in a controlled browser and produces the PageSpeed score and lab metrics.
ConceptPageSpeed score — Lighthouse Performance score
The 0–100 number PageSpeed Insights shows — a weighted blend of five lab metrics, not a single measurement.
ConceptPreload scanner
The part of the browser that races ahead through the raw HTML to start downloads early — and the reason a hero image it cannot see arrives late.
ConceptRender-blocking resources
CSS and JavaScript in the <head> that the browser must fetch and process before it can paint anything — the most common cause of a slow first paint.
ConceptResource hints
The link tags that ask the browser to do network work early: dns-prefetch, preconnect, prefetch. Most audited sites ship some and still miss a connection to an origin they load from.
ConceptReverse proxy
A server that sits in front of your website, receives each visitor request, and can transform the response on the way out — how WebSpeed optimizes a page without touching your site.